Enumerator new and yield

At ruby docs I found this piece of code:

fib = Enumerator.new do |y|
  a = b = 1
  loop do
    y << a
    a, b = b, a + b
  end
end

Thing is i cant understand what it does. In general it calculates fibonacci's numbers, but I have hard time understanding the details. What is y (Enumerator::Yielder)? The docs says nothing about it. What does << method do? (I know its yield alias). Why does infinite loop happen when y << a is removed?Thanks!